Polyethylene protective yarn

ABSTRACT

High strength polyethylene yarns useful in ballistic-resistant, cut-resistant and other applications, fabrics produced from these yarns and the methods by which the yarns and fabrics are made. An untwisted yarn of the invention comprises a plurality of filaments in essentially parallel array and from about 0.5 to 5 weight percent of a water-dispersible binder material covering less than half the surfaces of the filaments. The yarn has a tenacity greater than about 17 g/d, a tensile modulus greater than about 300 g/d, fewer than 20 entanglements/meter in a scoured state and a width less than given by the formula 
 
W≦0.055√{square root over (d)}
where W is the yarn width in millimeters under a tensile load of 0.01 g/d measured on a flat surface and d is the yarn denier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved, high strength polyethylene yarnsuseful in ballistic-resistant, cut-resistant and other applications,fabrics produced from these yarns and the methods by which the yarns andfabrics are made.

2. Description of the Related Art

Among the requirements that protective clothing such as personal bodyarmor, chain saw chaps, and others must meet, in addition toballistic-resistance and/or cut resistance, are comfort and flexibility.Multiple layers of woven fabrics consisting of high strength and highmodulus fibers are commonly used in such protective clothing.

The preparation of high strength, polyethylene filaments and/ormulti-filament yarns has been described for example in U.S. Pat. Nos.4,411,854, 4,413,110, 4,422,993, 4,430,383, 4,436,689, 4,455,273,4,536,536, 4,545,950, 4,551,296, 4,584,347, 4,663,101, 5,248,471,5,578,374, 5,736,244, 5,741,451, 5,972,498 and 6,448,359 B1.Ballistic-resistant articles prepared from such high strengthpolyethylene filaments have been described for example in U.S. Pat. Nos.4,403,012, 4,457,985, 4,623,574, 4,650,710, 4,737,401, 4,737,402,4,748,064, 4,883,700, 4,916,000, 5,061,545, 5,160,776, 5,167,876,5,175,040, 5,187,023, 5,196,252, 5,343,796, 5,376,426, 5,440,965,5,480,706, 5,677,029, 5,788,907, 5,804,015, 5,958,804, 6,003,424, and6,276,254 B1.

U.S. Pat. No. 4,403,012 indicates that the fibers may be formed into afabric by any of a variety of conventional techniques. U.S. Pat. No.4,737,401 broadly indicates that plain woven, basket woven, satin andcrow feet woven fabrics, etc., can be made from high strengthpolyethylene filaments. However, to efficiently use conventional weavingequipment, the yarns to be woven must have some minimum degree of yarncoherence to avoid snags and wild loops which effect fabric quality andmay stop the loom. Weaving is also enhanced when the yarn to be woven isessentially round in cross-section and does not flatten when passingover guides. On the other hand, for maximum ballistic-effectiveness itis desirable that the yarns in woven fabrics are flat and are spread outinto thin layers.

Methods to achieve yarn coherence have included twisting, jetentanglement, and application of sizing material. Twisting improves theroundness of yarn bundles but it is known that twisting reduces theballistic effectiveness of fabrics produced from these yarns. This maybe in part because twisting induces stress in the yarns and in partbecause twisting prevents the woven yarns from spreading into thinlayers.

Air jet entanglement of yarn filaments as taught, for example, by U.S.Pat. No. 5,579,628, provides yarn coherence and improvesballistic-resistance as compared to twisted yarns. However, air jetentanglement may also damage the yarn and is an expensive process inboth capital costs for air compressors and in operating costs for energyconsumption.

Sizing of a plain weave fabric made from untwisted high strengthpolyethylene filaments with polyvinyl alcohol has previously beendescribed in U.S. Pat. No. 4,737,401. A process that covered virtuallyall yarn surfaces of synthetic filament yarns with sizing has beendescribed in U.S. Pat. No. 4,858,287.

Each of the methods and yarns cited above represented improvements inthe state of their respective arts. Nevertheless, none described thespecific constructions of the yarns and fabrics of this invention andthe methods by which they are achieved.

SUMMARY OF THE INVENTION

The invention is an untwisted polyethylene yarn comprising: a pluralityof filaments in essentially parallel array and from about 0.5 to 5weight percent of a water-dispersible binder material covering less thanhalf the surfaces of said filaments. The yarn has a tenacity greaterthan about 17 grams/denier (g/d) and a tensile modulus (modulus ofelasticity) greater than about 300 g/d as measured by ASTM D2256, fewerthan 20 entanglements/meter in a scoured state and has a widthsatisfying the following formulaW≦0.055√{square root over (d)}where W is the yarn width in millimeters under a tensile load of 0.01grams per denier measured on a flat surface, and d is the yarn denier.The requirement for the yarn width expressed by the above formulainsures sufficient yarn roundness for good weaving capability.

The invention is also a protective fabric comprising in majority portionthe yarn described above.

The invention is also an improvement to a process for the preparation ofuntwisted polyethylene yarns comprising a plurality of essentiallyparallel filaments, said yarns having a tenacity greater than about 17g/d, a modulus greater than about 300 g/d, and fewer than 20entanglements/meter. The improvement comprises applying about 0.5 to 5wt. % of a water-dispersible binder material so as to cover less thanhalf the surfaces of the filaments during a last drawing step under atension greater than about 2 grams/denier (g/d).

The invention is also an improvement to a process for the preparation ofa very low creep, ultra high modulus, low shrink, high tenacitypolyethylene multiple filament yarn, comprising:

-   -   a) drawing a high molecular weight polyethylene yarn at a        temperature within 10° C. of its melting temperature to form a        drawn, highly oriented polyethylene yarn;    -   b) then poststretching the yarn at a drawing rate of less than        about 1 second⁻¹ at a temperature within 10° C. of its melting        temperature, and cooling the yarn under tension sufficient to        retain its highly oriented state.        The improvement comprises applying to the yarn about 0.5 to 5        wt. % of a water-dispersible binder material so as to cover less        than half the surfaces of the filaments during one of drawing        step a) or poststretching step b) under a tension greater than        about 2 grams/denier.

The invention is also a process for the preparation of a protectivefabric comprising the steps of: weaving a fabric comprising in majorityportion the yarn described above; scouring the fabric to remove thewater-dispersible binder material and flattening the yarn.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing figures:

FIG. 1 illustrates a method of measuring the width of a yarn.

FIG. 2 illustrates a process for producing a yarn of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An objective of this invention is the provision of a high strength, hightensile modulus polyethylene yarn having a coherence and roundnesssuitable for weaving into a protective fabric and which flattens andspreads out when the fabric is scoured. The invention is an untwistedpolyethylene yarn comprising a plurality of filaments in essentiallyparallel array and from about 0.5 to 5 weight percent of awater-dispersible binder material covering less than half the surfacesof said filaments. The yarn has a tenacity greater than about 17 g/d, atensile modulus greater than about 300 g/d, fewer than about 20entanglements/meter in a scoured state and has a width satisfying thefollowing formulaW≦0.055√{square root over (d)}where W is the yarn width in millimeters under a tensile load of 0.01grams per denier measured on a flat surface, and d is the yarn denier.The requirement for the yarn width expressed by this formula insuressufficient yarn roundness for good weaving. Preferably, the yarn width(W) satisfies the following formulaW≦0.040√{square root over (d)}where W is the yarn width in millimeters under a tensile load of 0.01grams per denier measured on a flat surface, and d is the yarn denier.The woven fabric is especially useful in applications requiringballistic-resistance and/or cut resistance, more preferably the former.

FIG. 1 illustrates the method of yarn width measurement. A length ofyarn 10 is attached at each end to weights 30 and placed across a flatplate 20. At least 7 cm of the yarn are in contact with the flat plate.The flat plate is conveniently chosen to be the stage of an opticalmicroscope. The weights 30 are chosen in relation to the yarn denier soas to produce a tension in the yarn of 0.01 g/d. The width W of the yarnbundle lying on the flat plate 20 is measured by appropriate means, suchas an optical microscope, averaging at least five measurements atdifferent points along a 5 cm length.

When the yarn has some degree of twist, the yarn width W is measuredalong a yarn length at least twice the twist periodicity, and is themaximum (as opposed to average) measurement taken along this length.

The number of entanglements per meter is measured after scouring theyarn to remove the binding material. “Entanglements” are interlockedfilaments that cannot be readily separated. Entanglements may be formedduring the process of spinning multiple filaments. The number ofentanglements/meter is measured by the method of ASTM D4724-99, with themodification that the apparatus used is the Model HW70735 InterlaceTester manufactured by Industrial Machine Works, Waynesboro, Va.Preferably, the yarn of the invention has fewer than about 10entanglements/meter in the scoured state.

The percentage of the filament surfaces that are covered by thewater-dispersible binder material is determined using a microscope withdigital image analysis software, such as IMAGE-PRO® software from MediaCybernetics, Silver Spring, Md. To aid in the measurement, the bindermaterial may be selectively dyed to enhance contrast by using awater-soluble dye that is not absorbed by polyethylene.

The water-dispersible binder material is preferably selected from thegroup consisting of: a salt of an acrylic copolymer, sodiumcarboxymethyl cellulose, polyethylene oxide, polypropylene oxide,ethylene oxide/propylene oxide copolymers, polyvinyl alcohol, modifiedstarch, esterified starch, cationic starch, starch-styrene/butadienecopolymer, and mixtures thereof. Preferably, the water-dispersiblebinder forms about 0.5 to about 3 wt. % of the yarn of the invention. Itwill be understood that the terms “weight percent” or “wt. %” have theconventional meaning of weight of binder per weight of filaments plusbinder.

The untwisted yarn of the invention is produced by an improvement to aprocess for the preparation of polyethylene yarns having a plurality offilaments in essentially parallel array, a tenacity greater than about17 g/d, a tensile modulus greater than about 300 g/d and fewer thanabout 20 entanglements/meter in a scoured state. The improvementcomprises the application of about 0.5 to 5 wt. % of a water-dispersiblebinder material so as to cover less than half the surfaces of saidfilaments during a last drawing step under a tension of greater thanabout 2 grams/denier, more preferably under a tension of greater thanabout 3 grams/denier. Preferably, the last drawing step is at anelevated temperature between about 110° C. and about 160° C.

Surprisingly, the application of the binder material when the yarn isunder substantial tension is believed to be a key factor in achievingsuperior ballistic effectiveness in fabric woven from the yarn. Withoutbeing held to a particular theory of why the invention works, it isbelieved that application of the binder material when the yarn is undersubstantial tension prevents complete wetting of the surfaces of thefilaments. The binder forms limited area binding points betweenfilaments sufficient to provide cohesion to the yarn for weaving, butnot sufficient to reduce ballistic effectiveness. Moreover, the limitedarea binding points are more readily removed by scouring to achievemaximum ballistic effectiveness.

An untwisted polyethylene yarn having a plurality of essentiallyparallel filaments, a tenacity greater than about 17 g/d, a tensilemodulus greater than about 300 g/d and fewer than about 20entanglements/meter is preferably produced by any of the processesdescribed by U.S. Pat. Nos. 4,413,110, 4,551,296, 4,663,101, and6,448,359 B1, all incorporated herein by reference to the extent notincompatible herewith.

FIG. 2 illustrates one embodiment of the process of the invention.Ultra-high molecular weight polyethylene and mineral oil are charged toa mixer 10 maintained at elevated temperature. The partially dissolvedpolyethylene is passed to a screw extruder 20 which may be a singlescrew extruder or a twin screw extruder wherein the formation of apolyethylene solution is completed. Solution filaments 30 are spunthrough an air gap into a water quench bath 40 wherein the solutionfilaments are cooled and solidified to gel filaments. The solutionfilaments may be stretched on passing through the air gap to the quenchbath. The gel filaments are passed in sequence through a washer cabinet50 in contact with a low boiling extraction solvent to remove themineral oil and then through a drying cabinet 60. The gel filaments maybe stretched between the quench bath and the washer cabinet and throughthe washing and drying cabinets. The extracted and dried multi-filamentyarn is passed continuously from the drying cabinet over a driven heatedgodet 70 and associated idler roll 76, through a first heated tube 77and onto a second driven heated godet 78, and associated idler roll 79,operating at higher speed. The yarn is thereby stretched in the heatedtube 77. The yarn next passes under a tension greater than about 2 g/din kissing contact with an applicator roll 81 partially immersed in anaqueous solution 80 of a binding agent. The yarn containing the bindingagent is dried and stretched again on passing through heated tube 82 todriven heated godet 83 and associated idler roll 84 operating at higherspeed than associated rolls 78 and 79. After the last elevatedtemperature stretch, the yarn is passed under tension over a driven coldgodet 85 and associated idler roll 86 and collected without twist on awinder 90. The heated godets and heated tubes are typically attemperatures between about 110° C. and about 160° C. As used herein, theterm “elevated temperature” means a temperature within that range.

The untwisted yarn so produced has filaments in essentially parallelarray, a tenacity greater than 17 g/d, a tensile modulus greater thanabout 300 g/d, fewer than 20 entanglements per meter in a scoured state,about 0.5 to 5 vol % of a water-dispersible binding agent covering lessthan half the surfaces of the filaments, and a width in millimeters lessthan given by 0.055 times the square root of the yarn denier.

Preferably the yarn of the invention is produced by an improvement tothe process of U.S. Pat. No. 5,741,451, incorporated herein by referenceto the extent not incompatible herewith. This process comprises thepreparation of a very low creep, ultra high modulus, low shrink, hightenacity polyethylene multiple filament yarn by: a) drawing a highmolecular weight polyethylene yarn at a temperature within 10° C. of itsmelting temperature to form a drawn, highly oriented polyethylene yarn;b) then poststretching the yarn at a drawing rate of less than about 1second⁻¹ at a temperature within 10° C. of its melting temperature, andcooling said yarn under tension sufficient to retain its highly orientedstate. The improvement comprises applying to the yarn about 0.5 to 5 wt.% of a water-dispersible binder material so as to cover less than halfthe surfaces of the filaments during one of drawing step a) orpoststretching step b) under a tension greater than about 2grams/denier.

The protective woven fabric of the invention, preferablyballistic-resistant, comprises in majority portion an untwistedpolyethylene yarn comprising: a plurality of filaments in essentiallyparallel array and about 0.5 to 5 wt. % of a water-dispersible bindermaterial covering less than half the surfaces of said filaments. Theyarn has a tenacity greater than about 17 g/d and a tensile modulus(modulus of elasticity) greater than about 300 g/d as measured by ASTMD2256, fewer than 20 entanglements/meter in the scoured state and awidth satisfying the following formulaW≦0.055√{square root over (d)}where W is the yarn width in millimeters under a tensile load of 0.01grams per denier measured on a flat surface, and d is the yarn denier.

The woven fabric of the invention may be plain woven, basket woven,satin or crowfeet woven or any other standard weave. It is preferredthat the yarns in the fabric have as few out-of-plane bends as possible.An eight-harness satin weave is particularly preferred.

It is also preferred that the fabrics of the invention are scouredand/or calendered to flatten and spread the yarns, thereby enhancingtheir ballistic-resistance. It is most preferred that the fabrics of theinvention are both scoured and calendered, with calendering preferablyoccurring after scouring.

The ballistic-resistant woven fabric of the invention possesses at least5% greater specific energy absorption when impacted with a 9 mm FMJbullet at its V50 velocity than a woven fabric having the sameconstruction using polyethylene yarns having the same tenacity andtensile modulus but having more than 20 entanglements/meter and/orgreater twist.

COMPARATIVE EXAMPLE 1

The widths were measured of commercially available untwisted highstrength, high modulus polyethylene yarns. Table I below sets forth theyarn deniers and the measured yarn widths in comparison with 0.055 timesthe square root of the yarn denier. Each of these yarns had about 8entanglements/meter. TABLE I Yarn Width, 0.055{square root over(denier)}, Yarn Denier mm mm 1200 2.5 1.91 650 1.7 1.40 375 2.8 1.07 2152.1 0.81

It is seen that each of the prior art, untwisted yarns had yarn widthsthat exceeded 0.055 times the square root of the yarn denier.

COMPARATIVE EXAMPLE 2

A 60 filament, 650 denier highly oriented polyethylene yarn having atenacity of 30 g/d, a tensile modulus of 970 g/d, and a main meltingpoint of 147° C., as measured by differential scanning calorimetry (DSC)at a heating rate of 10° C./min, was prepared by the process of U.S.Pat. No. 4,663,101.

A number of packages of this yarn were post-stretched by the process ofU.S. Pat. No. 5,741,451. The yarn packages were placed on a creel andfed from the creel over a set of driven rolls into a post-stretchingoven at a temperature of 156° C. and thence to a second set of drivenrolls operating at a speed 2.63 times faster than the first set. Theyarns were thereby stretched 2.63:1 between roll sets at a temperaturewithin 10° C. of their melting point. The plurality of yarns leaving thesecond set of driven rolls was passed through a second post-stretchingoven at temperature of 154° C. to a third set of driven rolls operatingat a speed 1.2 times faster than the second set. The yarns passingthrough the second post-stretching oven were thereby stretched anadditional 1.2:1. Yarn tension between the second and third sets ofdriven rolls was 4 g/d. Each yarn leaving the third set of driven rollswas cooled under a tension of 2 g/d and then wound on individualpackages.

The wound yarns consisting of 60 essentially parallel filaments were of215 denier, having a tenacity of 38 g/d, a tensile modulus of 1320 g/d,a main melting point of 148° C. as measured by DSC, no twist, 8entanglements/meter and a width measured under a tension of 0.01 g/d of2.1 mm. As the yarn width of 2.0 mm exceeded 0.055 times the square rootof 215 (0.81 mm), and as the yarn contained no binder material, this wasnot a yarn of the invention.

Some packages of these yarns were put aside for later twisting (seeComparative Examples 3 and 4). Other packages of these yarns wererewound onto a warp beam and placed on a loom manufactured by LindauerDORNIER GmbH. Still other packages of these same yarns were used for theweft. An attempt to weave a plain weave fabric produced many snags,tight ends and operating difficulties. A plain weave fabric wasnevertheless prepared having wild filaments, slubs and irregular yarnspacings. On average the fabric had 17.7 warp and weft yarns percentimeter, an areal density of 88 g/m² and a thickness of 0.15 mm.Forty-two sheets of this fabric were plied up to an areal density of3.69 kg/m² and subjected to ballistic testing by NIJ Standard 0101.03using a 9 mm 124 grain FMJ bullet. According to this method, samples areplaced on a clay backing, and shot 16 times. The protective power of thesample is expressed by citing the impacting velocity at which 50% of theprojectiles are stopped. This is designated the V50 velocity. Thespecific energy absorption (SEA) is the kinetic energy of the projectileat the V50 velocity in Joules, divided by the areal density of thesample, kg/m². SEA has units of J-m²/kg.

COMPARATIVE EXAMPLE 3

Some of the same 60 filament, 215 denier yarns prepared in ComparativeExample 2 were twisted to 1.2 turns/cm on a MEADOWS Model 805-M ringtwister. The twisted yarns were used as the warp and weft of a plainweave fabric having 17.7×17.7 yarns/cm. No difficulty was experienced inthe weaving operation. The woven fabric had an areal density of 88 g/m²and a thickness of 0.15 mm. The fabric was cut into 46 cm squares,stacked to an areal density of 3.67 kg/m² and subjected to ballistictesting by NIJ Standard 0101.03 using a 9 mm 124 grain FMJ bullet. TheV50 velocity was 378 meters/sec and the specific energy absorption was32.0 J-m²/kg.

COMPARATIVE EXAMPLE 4

Some of the same 60 filament, 215 denier yarns prepared in ComparativeExample 2 were twisted to 2 turns/cm on a MEADOWS Model 805-M ringtwister. The twisted yarns were used as the warp and weft of a plainweave fabric having 22×22 yarns/cm. No difficulty was experienced in theweaving operation. The woven fabric had an areal density of 111 g/m² anda thickness of 0.17 mm. The fabric was cut into 46 cm squares, stackedto an areal density of 3.67 kg/m² and subjected to ballistic testing byNIJ Standard 0101.03 using a 9 mm 124 grain FMJ bullet. The V50 velocitywas 421 meters/sec and the specific energy absorption was 39.8 J-m²/kg.

EXAMPLE OF THE INVENTION

Sixty-filament polyethylene yarns were produced exactly as described inComparative Example 2 with the exception that before entering the secondpost-stretching oven and after passing over the second set of drivenrolls, the yarns, while under a tension of 4 g/d, made kissing contactwith a roll rotating in a 7.5 wt. % aqueous emulsion of PENFLEX™ starchstyrene butadiene copolymer from Penford Products Co., Cedar Rapids,Iowa. The yarns were dried and post-stretched and in the secondpost-stretching oven under the same conditions as in Comparative Example2, cooled under 2 g/d tension and wound on individual rolls.

The untwisted polyethylene yarns of the invention consisted of sixtyessentially parallel filaments and about 2.5 wt. % of water-dispersiblebinder material covering less than half the surface area of thefilaments. The yarns were of 220 denier, had a tenacity of 37 g/d, atensile modulus of 1290 g/d, a main melting point of 148° C. as measuredby DSC, no twist, 8 entanglements/meter in a scoured state, and a widthof 0.58 mm measured under a tension of 0.01 g/d. The yarn width was lessthan 0.055 times the square root of the denier.

A plain weave fabric having 17.7 warp and weft yarns per centimeter, anareal density of 90 g/m² and a thickness of 0.15 mm was readily wovenfrom these yarns without difficulty. Sheets of this fabric were plied upto an areal density of 3.69 kg/m²and subjected to ballistic testing byNIJ Standard 0101.03 using a 9 mm 124 grain FMJ bullet. The V50 velocitywas 445 meters/sec. SEA was 44.3 J-m²/kg.

The V50 velocity of this fabric of the invention was 17.7% greater andthe SEA was 38% greater than for the fabric of Comparative Example 3having the same construction, and woven from twisted yarns having thesame tenacity and tensile modulus. Suprisingly, the V50 velocity of thisfabric of the invention was also 5.7% greater and the SEA was 11%greater than for the finer weave fabric of Comparative Example 4, alsowoven with twisted yarns.

Having thus described the invention in rather full detail, it will beunderstood that such detail need not be strictly adhered to but thatfurther changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoined claims.

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 9. In a process for thepreparation of untwisted polyethylene yarns comprising a plurality offilaments in essentially parallel array, said yarn having a tenacitygreater than about 17 g/d, a tensile modulus greater than about 300 g/d,and fewer than 20 entanglements/meter: the improvement comprisingapplying about 0.5 to 5 wt. % of a water-dispersible binder material soas to cover less than half the surfaces of the filaments during a lastdrawing step under a tension of greater than about 2 grams/denier. 10.In a process for the preparation of a very low creep, ultra highmodulus, low shrink, high tenacity multifilament polyethylene yarn by:a) drawing a high molecular weight polyethylene yarn at a temperaturewithin 10° C. of its melting temperature to form a drawn, highlyoriented polyethylene yarn, b) then poststretching said yarn at adrawing rate of less than about 1 second⁻¹ at a temperature within 10°C. of its melting temperature, and cooling said yarn under tensionsufficient to retain its highly oriented state; the improvementcomprising applying to the yarn about 0.5 to 5 weight percent of awater-dispersible binder material so as to cover less than half thesurfaces of the filaments during one of drawing step a) orpoststretching step b) under a tension greater than about 2grams/denier.
 11. The process of claim 9 wherein the water-dispersiblebinder material is a member selected from the group consisting of a saltof an acrylic copolymer, sodium carboxymethyl cellulose, polyethyleneoxide, polypropylene oxide, ethylene oxide/propylene oxide copolymers,polyvinyl alcohol, modified starch, esterified starch, cationic starch,starch-styrene/butadiene copolymer, and mixtures thereof.
 12. Theprocess of claim 10 wherein the water-dispersible binder material is amember selected from the group consisting of a salt of an acryliccopolymer, sodium carboxymethyl cellulose, polyethylene oxide,polypropylene oxide, ethylene oxide/propylene oxide copolymers,polyvinyl alcohol, modified starch, esterified starch, cationic starch,starch-styrene/butadiene copolymer, and mixtures thereof.
 13. A processfor the preparation of a ballistic-resistant fabric comprising the stepsof: a) weaving a fabric comprising in majority portion the yarndescribed by claim 1; and b) flattening and spreading the yarns in saidfabric by additionally applying one or both steps selected from thegroup consisting of scouring said fabric and calendering said fabric.14. The process of claim 13 wherein said flattening and spreading stepcomprises scouring said fabric.
 15. The process of claim 13 wherein saidflattening and spreading step comprises calendering said fabric.